For example, in base stations of mobile phones and transmitting stations of digital broadcasting, a highly accurate reference frequency signal (reference signal) required for synchronizing a timing and frequency of signal transmission is supplied using a reference frequency generator (reference signal generator). This type of reference frequency generators includes a reference frequency generator which controls a voltage controlled oscillator to synchronize an output signal with a highly accurate reference signal obtained from a GNSS receiver, and outputs the reference frequency signal.
This voltage controlled oscillator is configured to generate different frequencies according to an inputted control voltage. One example of such a voltage controlled oscillator is an analog type using a quartz crystal unit as a resonator.
With the above configuration, due to various causes such as a position of a GNSS satellite, an obstacle, an interfering radio wave, or any failure of a GNSS antenna, the GNSS receiver may fail to receive a signal from the GNSS satellite and generate the reference signal. Therefore, a reference frequency generator having a free-running control function for continuously outputting a reference frequency signal even when the reference signal can no longer be acquired is proposed. In a situation where the reference signal is successfully acquired, the reference frequency generator is capable of storing data which is used to control the voltage controlled oscillator. When the reference signal can no longer be obtained, the reference frequency generator performs a free-running control (holdover control) on the voltage controlled oscillator based on the stored data in the past, and thus a highly accurate reference frequency signal is successfully outputted for a long time.
Patent Document 1 discloses this type of reference signal generator, for example. The reference frequency generator of Patent Document 1 includes a PLL circuit using a digital filter with a long time constant and is configured to supply an analog DC voltage converted by a D/A converter, to a voltage control input terminal of a voltage controlled crystal oscillator. The reference frequency generator of Patent Document 1 is configured to be capable of controlling the voltage controlled oscillator when free-running, by correcting a frequency fluctuation due to a change with time.